In recent years, microalgae which can produce biofuels through photosynthesis using sunlight and carbon dioxide in the atmosphere without competing against the food production have received attention. Currently, large-scale cultivation of algae for the purpose of producing biofuels and useful substances is often carried out outside in the tropics and subtropics where the amount of solar radiation is high so that the photosynthesis can be conducted to the maximum. However, the amount of solar radiation is excessive during the day in such regions, and the strong light raises the temperature of the culture environment. As a result, the growth and metabolism of microalgae are adversely affected, and thus it cannot be considered that the cultivation of microalgae in the regions is conducted under an optimum condition.
A technique which solves the above problem is described in U.S. Pat. No. 7,980,024 (PTL 1). This publication describes that “by floating a photobioreactor for cultivation on a pond or lake, heat is transferred between the water outside the photobioreactor and the liquid medium in the reactor”. As a result, it becomes possible to prevent the temperature of the liquid medium in the photobioreactor for cultivation from rising high. Moreover, WO2008/153202 (PTL 2) describes “a reactor for photosynthesis which floats on the surface of water and which is made from a transparent, flexible material”.